The subject invention relates generally to digital data communications and more particularly to apparatus employing multiplexing to combine a plurality of digital data channels for transmission over a single transmission path. The invention provides apparatus for automatically varying the channel configuration and is particularly useful in conjunction with digital data modems.
In the prior art, digital data modem apparatus is known for interfacing with data processing apparatus at either end of a transmission channel such as a telephone line. Multiport modems are also known which provide a plurality of channels for communication with a similar plurality of channels of the associated data processing apparatus. The multiple channel information is multiplexed for transmission over an individual line. Multiplexing is typically accomplished by TDM (time division multiplexing) with bit-by-bit interlacing of channels. Of course, many methods of multiplexing are known and could be used according to the invention.
In initiating communication across a transmission line between modems, it is generally known in the prior art to provide a sequence of initializing signals, in what is sometimes referred to as a "handshaking" operation. Such signals may indicate when a data source actually wants to transmit data or wants a channel at its disposal ready for transmission. An example of the former alternative currently in use is RTS (request to send or ready for sending). An example of the second alternative is DTR (data terminal ready) or DSR (data set ready). RTS is present only while a transmission is in progress, while DTR will be present throughout the time that the data source is engaged in interactive communication with a device such as a CPU. It is also known to provide DCD (data carrier detect) and RLSD signals (receive line line signal detect). It is known not only to provide DCD in the presence of a data carrier from a communicating modem but also to set DCD low upon receipt of a coded inverse of the RTS signal (RTS). One commercially available modem incorporating the use of signals as described above is the Milgo 96 MM. Such signals may be particularly taken advantage of in accordance with the invention as will be presently described.
Known multiport modems contain the necessary circuitry for switching between various port configurations in response to commands set manually by an operator. Such channel allocation or port reconfiguration is useful when data traffic patterns differ relatively infrequently in a known manner. It is then possible to set up a mode switching schedule which requires the intervention of an operator from time to time to effect mode changes. Moreover the actions of two operations, one manually operating a modem at each end of the transmission line, must obviously be coordinated. To make more efficient use of the expensive telephone channels, it would be desirable to have a dynamic port reconfiguration capability wherein the data processing apparatus and modem system would cooperate to reconfigure ports rapidly and automatically within operator intervention.